A Pb—Sn alloy plating solution has been widely used until now for plating to electronic devices for which soldering was necessary or for the formation of a solder bump electrode (bump) to a semiconductor wafer, etc. However, the Pb—Sn alloy plating solution contains toxic Pb, so that there are many problems in a wastewater treatment, environmental conservation, or a soil and groundwater pollution from wastes of the semiconductors, etc. In recent years, for the purpose of reducing such a burden on the environment, researches have been carried out as a substitute for the Pb—Sn alloy plating solution containing lead, on a plating solution using a binary alloy which utilizes tin as a first element and uses silver, bismuth, copper, indium, antimony, zinc, etc., as a second element, or a multicomponent alloy to which a third element is further added, and among these, a Sn—Ag plating solution is now becoming a main stream as a Pb-free plating solution.
In an electroplating process using the Pb—Sn alloy plating solution, however, when an anode (an anode of the electroplating process) made of Pb—Sn is used, Pb2+ and Sn2+ ions are dissolved from the anode into the plating solution, so that the balance of the components of the Pb—Sn alloy plating solution is kept substantially constant. On the other hand, in the electroplating process using, for example, a Sn—Ag plating solution, when an anode made of Sn—Ag is used, Ag is gradually precipitated on the surface of the anode to coat the surface of the anode, so that Sn2+ ion is not replenished from the anode to the plating solution. Thus, a balance of components of the plating solution is changed, whereby it causes a problem in the electroplating process using an anode made of Sn—Ag in the Sn—Ag plating solution. Therefore, in the electroplating process using a Sn—Ag plating solution, an insoluble platinum-plated titanium plate, etc., is used as an anode.
Also, when an insoluble anode is used, for replenishing components of the plating solution, a method in which a metal tin is supplied by dissolving in the plating solution can be considered. According to this method, however, in the case of an alloy plating with a metal which is nobler than tin, a noble metal is precipitated on the surface of the metal tin by substituting it in the plating solution so that there is a problem that dissolution of the metal tin is suppressed. Thus, replenishment has generally been carried out by adding a tin salt solution in which essential components of the plating solution had previously been dissolved (for example, see Patent Document 1.).
However, as described in the above-mentioned Patent Document 1, in the method in which Sn2+ ion is replenished by injecting a tin salt solution, etc. (hereinafter referred to as an “element solution”) where essential components of the plating solution are dissolved therein, it must prepare the element solution to be injected, and the element solution must be injected while analyzing the components of the plating solution whereby control of the plating solution is difficult and also a large cost is necessary.
To overcome the above-mentioned problems, it has been investigated a method in which tin(II) oxide powder having an extremely high solubility in an acid or an acidic plating solution is directly added to the plating solution whereby the tin component in the plating solution is replenished (for example, see Patent Document 2.).